There is considerable interest among those in the fields of broadcasting and recording to reduce the amount of information required to transmit or record an audio signal intended for human perception without degrading its subjective quality. Analog signals with reduced informational requirements can be carried within narrower bandwidths, and digital signals with reduced information requirements can be carried at lower bit-rates.
One common technique used to reduce the informational requirements reduces the dynamic range of the audio signal to be transmitted or recorded. Dynamic range control is used to protect equipment from excessively high-amplitude signals and to achieve certain artistic results. In general, the overall goal of dynamic range control is to alter the dynamic range of an audio signal without introducing any other perceptible distortion.
In broadcasting, for example, the dynamic range of audio signals is "compressed" prior to broadcast to avoid overloading transmission equipment and/or to avoid severe audible distortion. Similar concerns apply to conventional tape and disc recordings.
Dynamic range "compression" reduces the dynamic range of an input signal by applying a gain factor less than one over a range of input signal amplitude. A "compressor" provides for signal amplitude compression. A "limiter" provides "limiting" which is a special case of compression, preventing the peak-amplitude of an input signal from exceeding a specified level by applying a very low gain factor to high-level signals.
A second common technique used to reduce the informational requirements of an audio signal reduces the amount of information used to represent or code the audio signal; however, as the amount of information is reduced, encoding inaccuracies increase and may become audible in the form of "coding noise." Coding noise degrades the subjective quality of the coded signal. So-called psychoacoustic-coding or perceptual-coding techniques attempt to reduce the informational requirements of an audio signal without introducing audible coding noise. Two examples of "split-band" perceptual-coding techniques are subband coding and transform coding. Perceptual-coding techniques exploit a characteristic of human heating; a stronger signal may mask or render inaudible a weaker signal if the two signals are sufficiently close in frequency. By splitting an audio signal into narrow frequency bands and independently coding the signal energy in each band, the aural effect of the coding noise is more likely to be inaudible because it is confined to the same frequency band as the coded spectral energy.
Coding systems which implement perceptual-coding techniques attempt to reproduce a representation of the input audio signal which preserves the perceived loudness of input signal spectral components. This is often accomplished by preserving some measure of spectral amplitude such as root-mean-square (RMS); however, many perceptual-coding systems lead to uncertainties in the peak-amplitude level of the reproduced signal. This uncertainty may include an increase in peak-amplitude, referred to herein as "peak-level increase" or PLI, which is tolerable in many coding applications where it is inaudible.
One application which uses both compression and informational reduction coding techniques is the Studio-Transmitter Link (STL) for broadcasting which delivers an audio signal originating from a studio to a broadcast transmitter. Typically, an STL includes a compressor and limiter which reduces the dynamic range and limits the peak-amplitude level of an audio signal, a perceptual-encoder which reduces the informational capacity requirements of the audio signal, a communications channel for delivering the encoded signal, and a perceptual-decoder for reproducing the compressed and limited audio signal for subsequent broadcast.
An STL is one example of an application which cannot tolerate excessive amounts of PLI. Because of PLI, the peak-amplitude of the signal reproduced by the perceptual-decoder may sometimes exceed the capabilities of the broadcast transmitter even though the peak amplitude of the audio signal input to the perceptual-encoder is properly limited. Transmitter overload resulting from PLI can create audible distortion and/or impermissible broadcast conditions.
Known techniques for controlling PLI include clippers, instantaneous gain reduction amplifiers, and conventional wide-band limiters. Unfortunately, these techniques introduce undesirable audible distortion in the reproduced signal. Clippers generate excessive harmonic distortion. Instantaneous gain reduction amplifiers, in effect, smear spectral components in the frequency domain. Conventional wide-band limiters reduce the perceived loudness of the reproduced signal.